In general, a transformer is an electrical device that transfers alternating current power from one electrical circuit to another by means of electromagnetic induction. In the transformer, voltage and current are proportional to a winding ratio of the primary winding and the secondary winding (V1:V2=N1:N2=1/I1:1/I2). In an ideal transformer, it is possible to obtain energy conversion efficiency of 100% where input power is equal to output power. However, in a practical transformer, various losses take place, and thus the energy conversion efficiency is lowered.
The losses occurring at the transformer are generally classified into two types: core loss and conduction loss. The number of turns in the winding is proportional to the applied voltage, but inversely to a cross-sectional area of a core used. Even when the core is wound with a proper number of turns, the core loss takes place depending on variation in magnetic flux density and an exponential function of frequency.
Generally, the core loss can be reduced by appropriately selecting a type, size, etc. of the core to be used, and designing the core loss and the conduction loss so as to keep their balances in a proper way. In most cases, the winding for the transformer is mainly made of copper, except for some cases where aluminum is used in a high-capacity transformer due to its weight. Therefore, while the voltage is applied to the transformer and the current flows along the copper winding, power loss having a quantity corresponding to I2R, i.e. the conduction loss called copper loss, takes place.
Further, when high frequency is applied to the winding of the transformer, its resistance value increases exponentially due to skin and proximity effects, and thus the conduction loss increases greatly. As a result, the conversion efficiency of the transformer is greatly lowered. Here, the skin effect refers to a phenomenon in which, when high-frequency current flows through a conductor, the current is concentrated on a surface of the conductor. Further, the proximity effect refers to a phenomenon in which, when high-frequency current flows through two parallel conductors, the current flows more intensively to portions of the conductors which are proximate to each other. At high frequency, the current concentrated on the surface of each conductor due to the skin effect leans to the opposite partial surfaces of the conductors due to the proximity effect.
FIGS. 1A and 1B illustrate a conventional single-layered one-turn transformer and a single-layered multi-turn transformer in a top plan view and a cross-sectional view, respectively.
In FIGS. 1A and lB, the left-hand figure is the top plan view, and the right-hand figure is the cross-sectional view that is taken along the line 140 of the left-hand figure. The single-layered transformers illustrated in FIGS. 1A and 1B are ones where primary windings 110 and 110′ and secondary windings 120 and 120′, which remove input and output terminals of voltage for the sake of convenience, are wound once (one turn) and multiple times (10 turns) around cores 130 and 130′, respectively. The multi-turn transformer of FIG. 1B is merely different in the number of turns in the windings, and is almost similar in function, compared to the one-turn transformer of FIG. 1A.
As can be seen from the cross-sectional views of FIGS. 1A and 1B, even when a sufficiently thick conductor is used as the winding, the current in the primary and secondary windings 110′ and 120′ flows along the opposite surfaces alone due to the above-mentioned skin and proximity effects, and thus the conversion efficiency of the transformers is lowered. This drawback has a more serious influence at high frequency.
For example, even when a copper plate of 10 mm thick is wound, a skin depth of the copper plate at a frequency of 20 KHz is no more than about 0.5 mm at a room temperature. Accordingly, the cross-sectional area of the copper plate over which the current actually flows in the primary or secondary winding of the transformer is merely 5% of the entire cross-sectional area. The copper plate corresponding to the remaining 95% does not perform any other function than cooling as an incidental effect without acting as the conductor. Here, the skin depth refers to an equivalent current penetration depth at the conductor (e.g. the conductive line) through which the entire current has to flow with the same loss. The skin depth has only functional relationship with frequency and properties of the conductive line, and is characterized in that it is inversely proportional to a square root of frequency.
It is a multilayered transformer that is proposed in order to improve low conversion efficiency in the single-layered transformers of FIGS. 1A and 1B.
FIG. 2 is cross-sectional view of a conventional multilayered transformer.
In FIG. 2, the multilayered transformer is illustrated in the cross-sectional view in which a plurality of windings is constructed in a multiple layer after the cross-sectional view of the single-layered multi-turn transformer of FIG. 1B is rotated at an angle of 90° in the counterclockwise direction. In other words, the multilayered transformer is to stack the single-layered transformer in a multiple layer. In the multilayered transformer of FIG. 2, input and output terminals of each of windings 210 and 220 are also removed for the sake of convenience. When the primary windings 210 and the secondary windings 220 are sequentially stacked as in the multilayered transformer of FIG. 2, the area of the opposite copper plates for the primary winding 210 and the secondary windings 220 is doubled, compared to each of the single-layered transformers of FIGS. 1A and lB, and thus the area of the copper plate through which the current flow actually is also twice. That is to say, the multilayered transformer of FIG. 2 can obtain double conversion efficiency over each of the single-layered transformers of FIGS. 1A and 1B. A winding method used in the multilayered transformer of FIG. 2 is called a sandwich winding method. This sandwich winding method is adopted by most transformer manufacturers, thus being widely used. However, in spite of the sandwich winding method that is widely used at present, in the case of a high-frequency transformer, there occur large losses at the transformer. In the case of a high-frequency, high-capacity transformer, owing to the use of a large-size core, a voltage of several thousands of volts can be applied even with 10 turns or less. Thus, considering that the conversion efficiency of the transformer is proportional to the number of turns, the high-frequency, high-capacity transformer has still low conversion efficiency.
For example, in the case of 1,000 KW transformer having inputs of 1,000 V and 1,000 A, assuming that it is designed to have 10 turns at a frequency of 20 KHz, the current of 1,000 A should be fed to both surfaces of one copper pipe. In this case, a width of the copper pipe for the proper conversion efficiency of the transformer increases considerably, which makes it next to impossible to manufacture the copper pipe. Especially, because the skin depth is inversely proportional to the square root of frequency, the higher the frequency becomes, the lower the conversion efficiency becomes. Consequently, in order to maintain the proper conversion efficiency, there occurs a serious problem in that the width of the copper pipe increases considerably.